Solve for $x$ and $y$ using substitution. ${3x+y = -9}$ ${y = 4x+5}$
Solution: Since $y$ has already been solved for, substitute $4x+5$ for $y$ in the first equation. ${3x + }{(4x+5)}{= -9}$ Simplify and solve for $x$ $3x+4x + 5 = -9$ $7x+5 = -9$ $7x+5{-5} = -9{-5}$ $7x = -14$ $\dfrac{7x}{{7}} = \dfrac{-14}{{7}}$ ${x = -2}$ Now that you know ${x = -2}$ , plug it back into $\thinspace {y = 4x+5}\thinspace$ to find $y$ ${y = 4}{(-2)}{ + 5}$ $y = -8 + 5$ $y = -3$ You can also plug ${x = -2}$ into $\thinspace {3x+y = -9}\thinspace$ and get the same answer for $y$ : ${3}{(-2)}{ + y = -9}$ ${y = -3}$